linux-kernel - Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device reset mechanism

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Message-ID: <1da0fa2c-8056-9ae8-6ce4-ab645317772d@metux.net>
Date:   Thu, 18 Mar 2021 18:51:10 +0100
From:   "Enrico Weigelt, metux IT consult" <info@...ux.net>
To:     Pali Rohár <pali@...nel.org>,
        ameynarkhede03@...il.com
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, alex.williamson@...hat.com,
        raphael.norwitz@...anix.com
Subject: Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device
 reset mechanism

On 15.03.21 00:55, Pali Rohár wrote:

> Moreover for mPCIe form factor cards, boards can share one PERST# signal
> with more PCIe cards and control this signal via GPIO. So asserting
> PERST# GPIO can trigger Warm reset for more PCIe cards, not just one. It
> depends on board or topology.

The pcengines apu* boards happen to be such candidates: they've got
three m.2 slots, but not all wired in the same way (depending on actual
model, not all have pcie wired). Reset lines are driven via gpio, and
some devices (I recall some lte basebands) sometimes need an explicit
reset in order to come up properly.

I have to check the schematics for the diffrent models, how exactly
these gpios are wired. (i've got reports that some production lines
don't have them wired at all - but couldn't confirm this on my own).

BTW: any idea how to inject board specific reset methods, after the
host brigde driver is already active ? In my case, apu boards, the
pci host bridge is probed via acpi and the apu board driver (which sets
up gpios, leds, keys, ...) comes much later.

--mtx

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Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info@...ux.net -- +49-151-27565287